Bent nails often result from poor hammering technique. However, they can also be caused by a dirty hammer face, especially when using cement-coated nails or working around adhesives. If you have problems, occasionally run a piece of fine sandpaper or emery cloth over the face. If you keep the face clean you will gain more solid contact with the nail and will avoid black marks on the wood caused by a dirty hammer.
To remove the popcorn ceiling, start by protecting all the surfaces including floors and walls that you do not want to get damaged or wet. Since water is used in the process be sure to turn off the electricity at the breaker. Use a garden sprayer or spray bottle if you don’t mind refilling often and saturate the ceiling. Use a plastic scraper to remove the popcorn material. As a side note if your ceiling was installed in the early seventies it may contain asbestos. Send a small sample to a certified lab prior to removing. While removing the popcorn, chances are some the joint compound was also removed. To insure a flat ceiling refill the joints and any screw/nail holes with fresh joint compound. Let dry, sand and repeat, to insure a flat surface a skim coat may need to be applied. Prime the ceiling with primer recommend for wallboard and a blank slate is created.